Centrosome precursors in the acentriolar mouse oocyte

Centrioles disappear from the mouse oocyte during early oogenesis. However, it has been known for some time that multiple structures known as microtubule organizing centers (MTOC) form the spindle poles during meiosis as well as the mitotic poles during early cleavage. The objective of this study was to identify and describe the structures which exist prior to the appearance of the multiple MTOC associated with meiotic division. Reported here for the first time is a description of the unique structures which exist before the onset of oocyte maturation, their location, and microtubule (MT) nucleating ability. Correlative confocal, immuno-, and electron microscopic studies of mouse oocytes released from ovaries directly into 2% paraformaldehyde (time zero) show two large gamma-tubulin-positive structures in the cortex, averaging 10 microm in diameter. The present work is the first to demonstrate that although these structures contain gamma-tubulin, they do not resemble MTOC morphologically nor do they appear to nucleate MT. They are termed here multivesicular aggregates (MVA), and ultrastructural analysis reveals that they contain a variety of vesicular structures including many ring structures of approximately 25 nm. At the onset of maturation, these two MVA migrate toward the GV breaking into smaller units, only some of which mature into MTOC and nucleate MT. These correlative microscopic studies support the conclusion that MVA are centrosomal precursors, but with a unique ultrastructure. The ultrastructural organization of MVA may explain the cryptic function of MTOC in the prematuration environment of the dictyate oocyte.